The behavior of the ring‐expansion homopolymerization of 2 (phenoxymethyl)thiirane (PMT) and propylene sulfide (PS), respectively, with thiazolidine‐2,4‐dione (TZD) as a cyclic initiator is investigated. The polymerizations show steadily growing molar masses with increasing monomer conversions. In addition, reversible merging reactions between rings are observed, with up to six merged macrocycles formed. The degree of merging is strongly dependent on the initial monomer concentration, whereas temperature has only a small impact. Under optimized conditions, ring‐poly(PMT) polymer with values of Mn up to 50 250 g mol?1 and dispersities down to 1.11 can be synthesized. DSC and ESI‐MS measurements of the novel ring‐poly(PS) prove the formation of ring polymer having topological purity above 95%.
